As integrated circuits are scaled to smaller and smaller dimensions, continued improvement in device drive current is necessary to maintain optimum transistor performance. In a metal-oxide-semiconductor (MOS) transistor, drive current is determined, in part, by gate length, gate capacitance, and carrier mobility. At a given device size, improved device current can be obtained by increasing the carrier mobility. A widely-used technique to enhance carrier mobility includes inducing strain in the active regions of the MOS transistors. Strain or stress in the crystalline lattice of the transistor substrate can enhance bulk electron and hole mobility through the crystalline lattice.
A common practice used to create strain, or stress, in a crystalline substrate is to form a layer of material in the substrate that has a lattice constant that differs from the substrate material. For example, strain can be induced in devices formed in a single crystal silicon substrate by forming regions of silicon germanium (SiGe) or silicon carbide (SiC). Since the lattice constant of SiGe is larger than that of silicon, the lattice mismatch puts the silicon under tension and the charge carrier mobility increases through the strained silicon lattice. Similarly, the lattice constant of SiC differs from silicon, however, the type of strain created by SiC differs from that created by SiGe. Alloys such as SiGe create compressive strain in silicon, while SiC creates tensile strain in silicon. A bi-axial, in-plane tensile strain field can improve performance in N-type MOS devices, and compressive strain can improve performance in P-type MOS devices. Further, other materials can be used to create strain in semiconductor substrates depending upon the particular substrate material and its lattice constant. For example, hetero-epitaxial processes can be used to form a wide range of materials, such as germanium (Ge) and silicon (Si) in III-IV substrates.
The fabrication of substrates having hetero-epitaxial regions is generally coupled with the use of advanced transistor materials to fabricate MOS devices having exceedingly small feature sizes. For example, such technology is employed to fabricate MOS devices having gate lengths on the order of 45 nm with continued scaling to 22 nm. Although hetero-epitaxial substrate regions and advanced materials technology are useful for the fabrication of extremely small devices, typical epitaxial processes produce large regions of epitaxial material. Such large area epitaxy can limit the fabrication of devices having feature sizes considerably less than 45 nm. A particular problem encountered with large area epitaxial deposition concerns plastic strain relaxation that takes place in the bulk epitaxial material. The relaxation reduces the difference in lattice constant between the epitaxial material and the substrate, which, in turn, reduces the strain imparted to the crystalline substrate.
Accordingly, improved technology is necessary for the utilization of hetero-epitaxial materials for the fabrication of transistor devices having extremely small feature sizes.